The present invention relates to a mass spectroscope which includes an ion trap section having a function of trapping ions, and is used to identify the composition of a substance. The invention also relates to a method for adjusting the mass spectroscope.
The ion trap section of the mass spectroscope is constituted of a plurality of electrodes each having a hyperboloidal cross-sectional shape. By applying a high-voltage high-frequency signal (hereinafter, referred to as “high-voltage RF signal”) and a direct current voltage to the electrode, an electric field is generated in the space formed by the plurality of electrodes, thereby trapping ions.
The principles of trapping ions by the ion trap section will be described with reference to FIG. 9. Here, a rod electrode section 905, which is configured by disposing in parallel four electrode columns (hereinafter referred to as “rod electrodes”) 908a-1, 908a-2, 908b-1, and 908b-2 each having a hyperboloidal cross-sectional shape, is taken as an example of the ion trap section. In addition, a circuit which is taken as an example of a high-voltage RF signal generating circuit includes: an RF signal source 901 which outputs a high frequency signal (hereinafter referred to as “RF signal”); and a resonance circuit 906 formed of coils 902a and 902b, capacitors 903a, 903b, and 904, a parasitic capacitor of wiring and the like.
On the assumption that with respect to the central axis of the four rod electrodes 908a-1, 908a-2, 908b-1, and 908b-2, an in-phase high-voltage RF signal is applied to one rod electrode pair 908a-1 and 908a-2 which face each other, whereas a reversed-phase high-voltage RF signal is applied to the other rod electrode pair 908b-1 and 908b-2, the motion equation of ions on the x-y plane which is orthogonal to the central axis is represented by the following equations:
                                                        ⅆ                              x                2                                                    ⅆ                              ξ                2                                              +                      2            ⁢            q            ⁢                                                  ⁢                          cos              ⁡                              (                                  2                  ⁢                  ξ                                )                                      ⁢            x                          =        0                            (                  Equation          ⁢                                          ⁢          1                )                                                                    ⅆ                              y                2                                                    ⅆ                              ξ                2                                              +                      2            ⁢            q            ⁢                                                  ⁢                          cos              ⁡                              (                                  2                  ⁢                  ξ                                )                                      ⁢            y                          =        0                            (                  Equation          ⁢                                          ⁢          2                )                                ξ        =                              ω            ⁢                                                  ⁢            t                    2                                    (                  Equation          ⁢                                          ⁢          3                )                                q        =                              8            ⁢                                                  ⁢            eV                                              mr              0              2                        ⁢                          ω              2                                                          (                  Equation          ⁢                                          ⁢          4                )            
Here, e is the quantity of electric charge of ions; V is the amplitude of the high-voltage RF signal; m is the mass number of ions; r0 is the radius of the inscribed circle which inscribes the space surrounded by the rod electrodes; ω is the angular frequency of the high-voltage RF signal; and t is the time.
In general, it is well known that in order to trap ions, the mass-to-charge ratio of which is m/e, into an ion trap, V and ω have only to be determined in such a manner that q≦0.908.
However, even if V and ω are determined as described above, manufacturing errors of the inductance of a coil and a capacitor connected to each rod electrode pair, and the like, may cause a difference in amplitude between the high-voltage RF signals applied to the rod electrode pairs respectively. In such a case, the motion equations (equations 1 and 2) are not satisfied, and therefore, there is a case where the efficiency of trapping ions decreases, or a case where ions having a desired mass-to-charge ratio cannot be trapped.
As a solution for solving this problem, JP-A-2001-332211 (Patent Document 1) discloses a linear ion trap apparatus, wherein an ion trap is constituted of four rod electrodes, each of the rod electrodes has a variable capacitor, and each variable capacitor is configured to be adjustable in such a manner that the high-frequency voltages become equivalent to one another.
FIGS. 10A and 10B are graphs each illustrating the relationship between the resonance frequency and the drive frequency measured when any of the variable capacitors which are connected to the rod electrodes respectively is adjusted to adjust the amplitude difference between the high-voltage RF signals. FIG. 10A illustrates frequency characteristics of the high-voltage RF signals measured when the frequency synchronizing unit makes the resonance frequency fR and the drive frequency fD equivalent to each other in a state in which the amplitude difference between the high-voltage RF signals is not corrected. It is revealed that there is a large difference in amplitude between the high-voltage RF signals of the rod electrode pairs at the resonance frequency. FIG. 10B illustrates the frequency characteristics measured when the amplitude difference between the high-voltage RF signals has been corrected by a variable capacitor from the state shown in FIG. 10A. It is revealed that although the amplitude difference decreases by the correction of the amplitude difference, the resonance frequency and the drive frequency are not equivalent to each other.
Therefore, in order to make the resonance frequency fR and the drive frequency fD equivalent to each other, it is necessary to further adjust another variable capacitor, which produces the problem of increasing the operation load of the adjustment. In addition, when the ion trap apparatus is operated in the state in which the resonance frequency and the drive frequency are not equivalent to each other, the amplification factor may decrease, which causes the power consumption to increase, or the operation margin of the circuit may decrease, which causes the ion trap apparatus to operate abnormally.